8.8 Accuracy of the Magnetic Heading System.
8.8.1 General.
a. The operating rules, such as part 91 and part 135, specify the minimum required equipment that must be installed in part 23 airplanes based on the operation, such as VFR or IFR.
Under VFR operation, part 91, § 91.205, requires a magnetic direction indicator (that is normally intended to be a compass) for heading information. Under IFR operation, part 91, § 91.205, requires a gyroscopically stabilized heading system. Section 23.1303(c), Amendment 23-62, amended the requirement from “A direction indicator (non-stabilized magnetic compass)” to “A magnetic direction indicator.” As new technology becomes more affordable for part 23 airplanes, many electronic flight instrument systems will use magnetically stabilized direction indicators (or electric compass systems) to measure and indicate the airplane heading to provide better performance.
b. The general airworthiness requirements in part 23, § 23.1301 and § 23.1525, determine the flight instrument and equipment accuracy requirements for part 23 airplanes. Part 23 does not prescribe specific accuracy requirements for magnetic gyroscopically stabilized heading systems. Specific accuracy requirements for avionics may be found in the related TSO and, as acceptable means of compliance to § 23.1301, in ACs, notices, or policy statements/letters.
8.8.2 Magnetic Non-Stabilized Direction Indicator. A magnetic non-stabilized direction indicator that is required by § 23.1303 should have an accuracy of 10 degrees or have a correction card, placard (reference § 23.1327), or a back-up gyroscopic direction indicator, provided the indicator is accurate better than 10 degrees. If the sole purpose of the gyroscopic direction indicator is for backing up the magnetic non-stabilized direction indicator, then the accuracy of presentation of headings can also be to 10 degrees. However, if a gyroscopic direction indicator is installed to meet the IFR operating rules, then the installation requirements are defined by § 23.1301.
8.8.3 Magnetic Gyroscopically Stabilized Direction Indicator. As installed, final accuracy for a magnetic gyroscopically stabilized direction indicator of ±4 degrees on the ground or ±6 degrees in normal level flight on any heading would meet the requirements of part 23, § 23.1301. This accuracy applies after compensation. It should include cumulative errors in all combinations due to several factors. These consist of the equipment itself, the current flow in any item of electrical equipment and its associated wiring, the movement of any part (for example, controls or undercarriage), and the proximity of any item of equipment containing magnetic material to the magnetic sensor.
8.8.4 Comparator Monitor.
a. For systems installations that include two magnetic gyroscopically stabilized heading systems and a comparator that monitors the differences between the headings of the two 23
systems, the comparator trip point, set as follows, would meet the requirements of part 23, § 23.1301.
(1) 6 degrees in stabilized level flight.
(2) 6 degrees plus one half of the bank angle; or
(3) 12 degrees with a bank angle greater than 6 degrees.
(4) The alert function can be disabled at a bank angle greater than 20 degrees.
(5) An alert is provided if the condition exceeds 60 seconds, but it allows two minutes for a turn error, as stated in the TSO.
b. The 6-degree trip point during level flight allows a heading error of as much as 12 degrees on one of the systems. This would comprise one system at the 6 degrees in-flight tolerance limit while the other system, presumably with some malfunction, could have an error of 12 degrees in the same direction before the comparator monitor alert is tripped.
8.9 Rate-of-Turn Instrument.
a. Under §§ 91.205 and 135.159, a rate-of-turn instrument is not required if a third attitude instrument usable through flight attitudes of 360 degrees of pitch-and-roll is installed following the instrument requirements prescribed in § 121.305(j). If required, place the rate-of-turn indicator near the heading indicator.
b. A second approved attitude indicator may substitute for the rate-of-turn instrument.
AC 91-75, Attitude Indicator, provides one method. This AC is applicable to part 23 certificated airplanes (or airplanes certificated under earlier equivalent regulations) that weigh less than 12,500 pounds and are operated under part 91. The second approved attitude indicator must be powered by an independent power source from the primary attitude indicator. This can be accomplished by using independent standby instruments, another independent PFD, or independent reversionary attitude display. Most of the installations with electronic displays that meet the requirements of § 23.1311 and this AC should comply with AC 91-75.
8.10 Slip-Skid Instrument. The slip-skid information is required by §§ 91.205(d)(4) and
135.159(b), as applicable. The FAA suggests locating the slip-skid display directly below or near the rate-of-turn instrument, if installed, or under or within the primary attitude display.
8.11 Vertical Speed Indicator. If provided, present the vertical speed indicator to the right or directly below the altitude indicator with a scale appropriate to the performance of the aircraft.
9.0 Powerplant Displays.
9.1 General.
a. This section defines a means of presenting powerplant performance and condition information about the airplane's operation. It also provides guidelines about when these functions should be displayed to the pilot. In general, there have been two methods used to accomplish this.
These methods include: (1) display raw engine parameters to the pilot for interpretation, or
(2) collect powerplant data and have an automatic monitoring system interpret and report the powerplant condition to the pilot. Use the following evaluation criteria when considering the installation of electronic powerplant displays.
b. There is a need to evaluate each airframe, engine, and airframe/engine interface with the operational characteristics of these systems to determine the primary powerplant parameter requirements. For this evaluation, and as used in this section, a primary powerplant parameter is needed to start the engine and to set and monitor engine power within powerplant limitations.
Appropriate procedures for operation of an integrated electronic powerplant display system should be in the AFM.
c. For multiengine airplanes, a failure or malfunction affecting the display or accuracy of any propulsion system parameter for one engine should not cause the loss of display or accuracy of any parameter for the remaining engine(s). If multiple propulsion parameters are integrated on one display, and the display fails, it is acceptable to provide a secondary propulsion parameter display.
9.2 Loss of Critical Powerplant Information.
a. No single failure, malfunction, or probable combination of failures, should result in either the loss of critical powerplant information or an erroneous display of powerplant parameters that would jeopardize continued safe flight and landing of the airplane. Usually, for engines that do not generally rely on the pilot to prevent exceeding their limits in normal operations or engines such as with electronic engine controls featuring engine limit protection, the loss of powerplant displays should not cause immediate jeopardy to continued safe flight and landing.
b. A secondary display providing powerplant parameters for cases of loss of a primary powerplant display may be used provided it is located so the pilot can adequately view the powerplant parameters.
c. Throttle or power lever position may be used in place of lost powerplant display parameters. This would apply if throttle position or power lever position provides a positive indication of the powerplant power level required to maintain safe flight to a landing, and if the throttle has a means to preclude exceeding powerplant operating limits.
9.3 Powerplant Information.
a. Presentation of primary powerplant parameters continuously when they are required unless the monitor provides an adequate alert of the function. Also, provide a manual select option for the pilot to present the information. A parameter defined as primary for engine start, but not for other normal engine operation, may only have to be presented continuously during engine start.
b. Before and upon reaching or exceeding any operating limit, the display should present the required powerplant parameters without pilot action. Timely alerts for each phase of flight should be provided when any operating limit is reached or exceeded for the required powerplant parameter. The alerts should be in a form that enables the pilot to identify and carry out the necessary and appropriate actions. The required powerplant information should be presented continuously during a critical takeoff and landing phase of flight to minimize pilot distraction until an established rate of climb or minimum altitude is achieved.
c. Displays that provide multiple powerplant parameters should be designed such that any parameter, display, or alert will not suppress another display or alert that also requires immediate crew awareness necessary to conduct safe operation of the aircraft and engine(s). Alerts that could cause subsequent activation of other displays or alerts should be presented in a manner and form to ensure appropriate identification and prioritization of all significant hazards and required crew actions. See section 18 for more guidance on annunciation.
9.4 Digital Reading Alphanumeric Displays.
a. Digital reading alphanumeric displays are most valuable when integrated with an analog display by adding a precise, quantitative indication to complement an analog display's qualitative indication. Proper integration should include a means for pilots to clearly associate the analog and digital indications (e.g., via close proximity). Digital reading alphanumeric powerplant displays formats should not be used in place of analog formats to indicate values of engine parameters where trend or rate-of-change information is important for safety, or when the pilot needs to monitor parameters with a quick glance. Digital reading alphanumeric displays limit the pilot's ability to assess trend information and result in reduced crew awareness. Digital reading alphanumeric displays are also limited in their ability to allow the pilot to easily compare parameters from multiple engines or to check the general proximity of differing parameters against their individual limits. While these shortcomings can be compensated for with more design provisions, use digital reading alphanumeric displays with caution and carefully evaluate each airframe, engine, and airframe/engine integration.
b. Section 23.1311(a)(6), Amendment 23-62, requires sensory cues that provide a quick glance sense of rate and, where appropriate, trend information to the parameter being displayed to the pilot. Section 23.1311(a)(7) requires incorporated equivalent visual displays of the instrument markings required by §§ 23.1541 through 23.1553 or visual displays that alert the pilot to abnormal operational values or approaches to established limitation values, for each parameter required to be displayed by this part.
c. An ELOS will be necessary for digital -reading alphanumeric only displays that are not associated with any scale, tape, or pointer. It may be difficult for pilots to determine the margin relative to targets or limits, or compare between parameters. To comply with § 23.1311(a)(7), a scale, dial, or tape will be needed to accomplish the intended pilot task. In order to demonstrate an equivalency, a human factor and/or pilot evaluations will be needed that will consider the following factors. These are subject to evaluation on a case-by-case basis.
(1) The visibility and relative location of the indicated parameter should be reviewed, including appropriate conditions of lighting and instrument panel vibration.
(2) The ability to assess necessary trend or rate-of-change information quickly, including when this information may be needed during in-flight engine restarts.
(3) The ability to assess how close the indicated parameter is relative to a limit.
(4) The value to the crew of quickly and accurately comparing engine-to-engine data for multiengine aircraft.
(5) Engine design features or characteristics that would forewarn the crew before the parameter reaches the operating limit (for example, redline).
(6) The use of color in accordance with § 23.1549 to indicate the state of operation: a green indication would indicate normal operation, a yellow indication would indicate operation in a takeoff or precautionary range, and a red indication would indicate operation outside of the safe operating limits.
9.5 Marking of Powerplant Parameters. Mark powerplant parameters on electronic displays in accordance with § 23.1549. AC 20-88A provides alternate methods of marking electronic powerplant displays. Alternate methods of marking the displays may be performed. However, the FAA may evaluate these markings on a case-by-case basis depending on each airframe, engine, integration, and appropriate human factors considerations. Alternate markings that do not comply with the requirements of § 23.1549 require an ELOS.
10.0 Electronic Displays for Navigation Information.
10.1 Guidance Information. Navigation information used by the pilot for steering commands and to monitor deviation from a navigation path should be in the pilot's primary FOV. Warnings and cautions should also be in the primary FOV. For more guidance on FOV, see section 15 and Table 3 of this AC.
10.2 Display Integration.
a. Navigation guidance information may be integrated with the PFDs. Common examples include HSIs that combine inputs from a directional gyro with a course deviation 27
indicator, or an FD integrated with the ADI. Additionally, information from more than one navigation source may be displayed separately or simultaneously.
b. If information from more than one navigation source can be presented, the selected source should be continuously indicated to the pilot. If multiple sources can be presented simultaneously, the display should indicate unambiguously what information is provided by each source and which one has been selected for guidance. Some airplanes are equipped with an autopilot and/or FD coupled to the lateral and vertical guidance system. On these airplanes, the input to the autopilot and/or FD should coincide with the navigation source selected on the PFD or primary navigational display.
10.3 Reversionary Navigation Displays. Reversionary requirements for navigation display information depend on the rules under which the aircraft is operated and the hazards associated with the loss of or misleading information from the display. The integration of non-navigation information (for example, traffic, weather or flight parameters) may affect the hazards associated with the loss of, or misleading information from, the display. In these cases, the applicant should perform a system safety assessment following AC 23.1309-1E. For more guidance on safety assessments, see AC 23.1309-1E and section 26 of this AC for more guidance.
10.4 Navigation Database. For area navigation equipment, the navigation database is an essential component of the equipment. Guidelines for the navigation database can be found in RTCA/DO-200A. See AC 20-138 and AC 20-153 for more guidance and information.
11.0 Aircraft Flight Manual (AFM). For equipment required for IFR approval, the AFM or supplemental AFM should contain the limitations and the operating and emergency procedures applicable to the equipment installed. Installations limited to VFR use only may require an AFM or supplemental AFM depending on the complexity of the installation and the need to identify necessary limitation and operating procedures. AC 23-8C contains more policy and guidance on AFMs.
12.0 Electronic Checklist and Charts. Policy and guidance on electronic checklist and charts on electronic displays are contained in AC 23-8C, AC 91-78, AC 120-64, and AC 120-76A.
13.0 General Human Factors Considerations for Design of Electronic Displays.
13.1 General. Electronic displays can provide many innovative display flexibilities and features that were not possible with traditional displays. Technology improvements in display design and integration can enhance pilot performance and improve situation awareness when innovations are designed using sound processes and appropriate design criteria that account for human performance. The applicant should ensure that human performance considerations are adequately addressed throughout the design process. Early FAA involvement and an effective working relationship between the FAA and the applicant will aid in the timely identification and resolution of human factors related issues. The FAA encourages the applicant to develop human performance considerations, including evaluation plans, and present them to the FAA early in the certification process.
13.2 Human Factors for Certification of the Part 23 Small Airplanes. Policy statement number PS-ACE100-2001-004 provides guidance for human factor certification plan or the human factor components of a general certification plan when one is submitted as part of a TC, STC, or ATC project. The application of this guidance will ensure that human factors issues are adequately considered and addressed throughout the certification program.
13.3 Cockpit/Flight Deck Design, GAMA Publication No. 10. The purpose of GAMA Publication No. 10 is to provide manufacturers of small aircraft and systems with human factors recommendations for the design of cockpits/flight decks and their associated equipment. This publication was developed as a cooperative effort between the FAA and industry.
13.4 SAEs and RTCAs Recommended Practices and Standards. Evaluation of the electronic display system should consider airworthiness regulations, recommended practices, and standards from industry documents as listed in section 3 of this AC.
13.5 Human Factors Compliance Considerations.
a. The applicant should identify all human factors related regulations that apply to the installation and operation of the display system or component being certified. The applicant should document how the system or component complies with each of these regulations. A partial list of relevant regulations is contained in policy statement PS-ACE100-2001-004. The FAA and the applicant should develop and agree on a plan to describe how the applicant intends to show compliance with the relevant regulations. It should contain enough detail to assure that human factors compliance considerations have been adequately addressed. The test plan should also allow enough time to accomplish all necessary testing agreed to by the applicant and the FAA.
b. The applicant should assure the following.
(1) The intended use of the system does not require any exceptional skill or generate any unreasonable workload for the pilot.
(2) The use of the display does not require unreasonable training requirements to adequately show that the pilot can understand and properly operate the system in all operational environments.
(3) The design characteristics of the display system should support error avoidance and management (detection, recovery, etc.).
(4) The display system is integrated and is consistent or compatible with other cockpit controls and displays, and it creates no more burden on the pilot regarding the new display system or operation of other systems.
(5) If a failure occurs, it does not prevent the pilot from safely operating the airplane.
c. Manufacturers should provide design rationale for their decisions regarding new or unique features in a display. Evaluation should be conducted to collect data in support of the new or unique features to assure they have no unsafe or misleading design characteristics. The design teams, including pilots, should concur with the conclusion found with the data collected.
d. The applicant should conduct display evaluations with several human factors and pilots representatives whom are intended users of the system. For systems that contain new or unique features, a greater number of evaluators should be considered. Allow a reasonable amount of training time to learn all required features of the display system. The evaluators should be familiar with the guidance contained in this AC before the evaluation. Evaluators should also be familiar with human factors display regulations, guidance, standards, and recommendations contained under sections 3.
13.6 Intended Function. One of the regulations applicable to displays is § 23.1301. As part of showing compliance with § 23.1301, the display system must be of a type and design appropriate to its intended function when used by representative general aviation pilots. An AFM, AFMS or Pilot’s Guide may be used to describe the intended function.
13.6.1 Demonstrate Compliance. To show compliance with § 23.1301, an applicant must show that the design is appropriate for its intended function. The applicant’s statement of intended function should be specific and detailed enough for the certification authority to evaluate the intended function(s) and to evaluate whether the associated pilot tasks (if any) can be adequately achieved. This is important for systems with a pilot interface component, as the certification authority should evaluate the intended function from the pilot’s perspective as well as from a systems perspective. There are several safety concerns, including the concern that the pilot could rely on a display that does not meet minimum safety standards. For example, applicants should elaborate on a statement that a new display system is intended to “enhance situation awareness” since a wide variety of different displays from terrain awareness, vertical profile, and even the primary flight displays, enhance situation awareness in different ways. Applicants should provide a greater level of detail to identify the specific aspect(s) of situation awareness that are to be enhanced and show how the design supports those aspects. Similarly, the terms “supplemental,” “non-essential,” “secondary,” etc., in isolation would not be acceptable as statements of intended function. While the statement of intended function must be submitted by the applicant, the certification authority will make the final determination of compliance to the rule, specifically on the acceptability of the proposed intended function.
13.6.2 Intended Function(s) and Associated Task(s).
a. List the intended function(s) and associated task(s) for the major pilot interfaces focusing on new or unique features that affect the pilot interface. A system may have multiple intended functions, provided each function is documented and all information depicted or indicated to the pilot supports one or more of the documented intended functions. See PS-ACE100 2001-004 paragraph 2.a., Intended Function, for more guidance.
b. You may use the following information to evaluate whether the statement of intended function and associated task is sufficiently specific and detailed.
(1) Does each feature and function include a statement of the intended function? Is there a description of the task associated with this feature/function?
(2) What assessments, decisions, or actions are required from the pilot for these intended functions?
(3) What other information is required for use with these intended functions (for example, other cockpit systems), assumed to be used in combination with the system?
(4) Is the operational environment in which these intended functions are to be used adequately described (for example, VFR, IFR, phase of flight, etc.)?
13.6.3 Method(s) of Compliance. The method(s) of compliance should be adequate to enable the certification authority to determine the following.
a. Is the system providing sufficient and appropriate information to support the intended functions and to achieve the associated tasks?
b. Is the level of detail, accuracy, integrity, reliability, timeliness, and update rate of the information matched appropriately to the task associated with the intended function?
c. Does the use of the system impair the pilot’s ability to use other systems or do other tasks?
d. Does the system or the use of the system impair the safe operation or the intended function of other systems or equipment?
13.6.4 Labeling. All displays and controls whose function is not obvious should be labeled and identified by function or operating limitations or any combination. The assumptions and/or limitations should be adequately documented in the AFM, AFM Supplement, and/or Pilot Operating Manual (as negotiated with the ACO). This applies to the manufacturer of the equipment and not to the installer. The installer is required to verify the intended function and to make any placards or flight manual limitations, according to subpart G of part 23, that the installed equipment makes necessary.
14.0 Location and Configuration of Displays.
14.1 Display Usability. Displays should be located such that the pilot(s) can monitor them with minimal head and eye movement between displays. Flight information should be legible, accurate, easily interpreted, sufficiently free of visual cutoff (viewing angle), parallax and distortion, for the pilot to correctly interpret it.
14.2 Basic T Configuration. The basic T-configuration should be used for airplanes certificated under § 23.1321, Amendment 23-14, or a later amendment. The basic T-configuration 31
is defined as an arrangement where the airspeed and altitude data are centered, respectively, directly to the left and right of the attitude data, with the direction data located directly below the attitude data.
14.3 Deviations from the Basic T Configuration.
a. Deviations from the basic T-configuration have been approved for individual instrument arrangements if the droop angle (angle below the § 23.1321(d) position) is 15 degrees or less, or if the elevated angle is 10 degrees or less. These angles are measured from a horizontal reference line that passes through the center of the attitude reference data with lines passing through the center of the airspeed and altitude data.
b. Unique displays or arrangements for attitude, altitude, airspeed, and navigation data, integration of combinations of these functions, or rearrangement of them from the basic T- configuration, may be approved when an equivalent level of safety, and a human factors evaluation are provided. This evaluation should consider the di fferent types of airplane operations, as defined by § 23.1559(b). Coordination with the Small Airpla ne Directorate is required. Deviations beyond these limits may be approved for individual flight instruments through a human factors evaluation and display installation evaluation, considering the following items:
(1) The display arrangement and its alignment to the normal line of the pilot's vision;
(2) The cockpit view;
(3) The integration of other functions within the displays;
(4) The data presented, format, symbology, etc., within the display; and
(5) The ease of manipulating controls associated with the displays.
15.0 Pilot Field-of-View Considerations.
15.1 General.
a. This guidance is specifically intended to address the visibility and placement of information used by the pilot. Past practice has typically involved FAA flight test pilots working with applicants to conduct qualitative assessments of the proposed equipment locations. This is intended primarily for new aircraft development programs, but it could also be used for extensive modifications to existing cockpits. Part 23 rules do not require the applicant to establish a cockpit design eye reference point from which to measure viewing distances and angular displacement to various cockpit equipments. However, we recommend you facilitate quantitative assessments of equipment locations for new instrument panel designs. These evaluations typically involve the pilot’s subjective assessment of the applicant’s cockpit to ensure it meets the intent of § 23.1321(a), which states the pilot should be able to use all the required instruments with “minimum head and eye movement.”
b. Many other factors need to be considered when determining the acceptability of displayed information in the cockpit. These factors can include the following:
(1) Readability;
(2) Clutter;
(3) Pilot scan pattern;
(4) Pilot workload;
(5) Parallax;
(6) Frequency of use and criticality;
(7) Attention-getting;
(8) Out-the-window correlation;
(9) Pilot disorientation; and
(10) Illumination and glare.
15.2 Primary Field-of-View. Primary optimum FOV is based on the vertical and horizontal visual fields from the design eye reference point that can be accommodated with eye rotation only.
With the normal line-of-sight established at 15 degrees below the horizontal plane, the values for the vertical and horizontal (relative to normal line-of-sight forward of the aircraft) are +/-15 degrees, as shown in figure 1. This area is normally reserved for primary flight information and high priority alerts. Table 3 also provides exam ples of information recommended for inclusion in this visual field. In most applications, critical information that is considered to be essential for safe flight, with warning or cautionary information that requires immediate pilot action or awareness, should be placed in the primary FOV.
15.3 Primary Maximum Field-of-View. Primary maximum FOV is based on the vertical and horizontal visual fields from the design eye reference point that can be accommodated with eye rotation and minimal head rotation. These values are +/-35 degrees horizontal, and +40 degrees up and -20 degrees down vertical, as shown in figure 1. These areas are normally used for important and frequently used information. A pilot’s visual scan and head rotation is minimized when information is placed in this area. Placement of information in this area also reduces the potential for spatial disorientation. Table 3 provides examples of information recommended for inclusion in this visual field.
Optimum Hori.?,:
NOM Vertical Field Optimum Horizontal 11/17/2011 AC 23.1311-1C Figure 1. Primary Field-of-View
15.4 Recommended Location for Displaying Information. Table 3 contains the recommended location for displaying information intended for new panel layout with integrated electronic displays; however, these guidelines should be followed for other installations as practicable. Deviations beyond these limits may be approved for individual flight instruments depending on the combination of factors listed in section 15.1. These deviations need a display installation evaluation. It may not be practicable for retrofit installations with new electronic displays, such as with ATC, STC, and field approvals, to comply with the values in the table. This is due to limitations in the systems and incompatible technologies between the aircraft and the system being added. In such cases, for any given item, the angular deviations should not increase from what was originally found to be acceptable, and the display characteristics should be at least as good as the original display. For retrofit installations, it may be acceptable for installations to fall outside the recommended data in table 3. These deviations may need an evaluation by the certification authority. Factors to consider during this evaluation include the distinguishing ability, attention-getting quality, readability, etc. The FOV angles should be applied for installation approvals where a design eye reference point exists. For installation approvals where no design eye reference point is defined, the linear panel distances from center in table 3 should be used.